Ch. 2 - Acids and Bases; Functional Groups

Wade9th EditionOrganic ChemistryISBN: 9780135213728Not the one you use?Change textbook

Wade 9th Edition

Ch. 2 - Acids and Bases; Functional Groups

Problem 28

Chapter 2, Problem 28

The C≡N triple bond in acetonitrile has a dipole moment of about 3.6 D and a bond length of about 1.16 Å. Calculate the amount of charge separation in this bond. How important is the charge-separated resonance form in the structure of acetonitrile?

Verified step by step guidance

To calculate the amount of charge separation in the C≡N bond of acetonitrile, use the formula for dipole moment: μ = q × d, where μ is the dipole moment, q is the charge separation, and d is the bond length.

Rearrange the formula to solve for q: q = μ / d.

Substitute the given values into the equation: μ = 3.6 D (Debye) and d = 1.16 Å (angstroms). Note that 1 D = 3.33564 × 10^-30 C·m and 1 Å = 10^-10 m.

Convert the dipole moment and bond length into consistent units (Coulombs and meters) before substituting them into the equation.

Discuss the importance of the charge-separated resonance form: The resonance form with charge separation contributes to the overall structure of acetonitrile, but the major contributor is the form without charge separation due to the stability of neutral molecules. The resonance form with charge separation is less stable due to the presence of formal charges.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Dipole Moment

The dipole moment is a measure of the separation of positive and negative charges in a molecule, indicating its polarity. It is expressed in Debye units (D) and reflects how unevenly the electrons are distributed. In acetonitrile, the dipole moment of 3.6 D suggests a significant charge separation due to the electronegativity difference between carbon and nitrogen.

Resonance Structures

Resonance structures are different ways of drawing the same molecule that illustrate the delocalization of electrons. In acetonitrile, the resonance forms show the potential charge separation, where the carbon atom can carry a positive charge while nitrogen carries a negative charge. These structures help in understanding the stability and reactivity of the molecule.

Bond Length

Bond length is the average distance between the nuclei of two bonded atoms. In acetonitrile, the bond length of the C≡N triple bond is approximately 1.16 Å, indicating a strong bond due to the overlap of atomic orbitals. Shorter bond lengths typically correlate with stronger bonds, which is crucial for understanding the stability of the molecule.

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Related Practice

Textbook Question

For each of the following compounds,

1. draw the Lewis structure.

2. show how the bond dipole moments (and those of any nonbonding pairs of electrons) contribute to the molecular dipole moment.

3. estimate whether the compound will have a large, small, or zero dipole moment.

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Textbook Question

Circle the functional groups in the following structures. State to which class (or classes) of compounds the structure belongs.

d. CH₃CONH₂

e. CH₃NHCH₃

f. RCOOH

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Textbook Question

For each of the following compounds,

1. draw the Lewis structure.

2. show how the bond dipole moments (and those of any nonbonding pairs of electrons) contribute to the molecular dipole moment.

3. estimate whether the compound will have a large, small, or zero dipole moment.

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Textbook Question

For each of the following compounds,

1. draw the Lewis structure.

2. show how the bond dipole moments (and those of any nonbonding pairs of electrons) contribute to the molecular dipole moment.

3. estimate whether the compound will have a large, small, or zero dipole moment.

a. CH₃CH=NCH₃

b. CH₃CH₂OH

c. CBr₄

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Textbook Question

Circle the functional groups in the following structures. State to which class (or classes) of compounds the structure belongs.

a. CH₂=CHCH₂COOCH₃

b. CH₃OCH₃

c. CH₃CHO

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Textbook Question

Circle the functional groups in the following structures. State to which class (or classes) of compounds the structure belongs.

(g)

(h)

(i)

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